1. Technical Field
The present invention is directed toward corrugators, and more particularly toward corrugators for making corrugated plastic tubing.
2. Background Art
A well known method for manufacturing corrugated pipes is to extrude a tube of thermoplastic from a head and then conform that tube to the interior of a corrugated mold tunnel formed by mold blocks. The mold blocks typically come in pairs and mate together to form a portion of the mold tunnel, and the thermoplastic is either blown into its shape (by pressure created within the mold tunnel) or by vacuum (where vacuum draws the air from around the mold tunnel).
Structures which have been used in the vacuum method of manufacture are illustrated in U.S. Pat. Nos. 4,319,872, 4,439,130, and 4,718,844. These patents all address the problem of drawing a uniform, strong vacuum around the entirety of the mold blocks which are continually moving during the molding process. As can be seen from these structures, it is difficult to inexpensively form any such mold block.
Certain problems naturally occurring in all such machines are exaggerated in machines which are used to make larger diameter tubes (for example, up to four feet in diameter and more).
Conforming the thermoplastic to the larger diameter mold blocks with a uniformity of wall thickness can be difficult.
Cooling of the larger mold blocks is also difficult. Of course, if the machine is used continuously over a long period of time, inadequate cooling of the mold blocks can result in improperly elevated temperatures through the molding process and degradation of the quality of the molded tubes.
Moving and properly aligning multiple mold blocks is also naturally more difficult with larger diameter mold blocks due to the mere size and weight of the parts involved and their interaction with other components. In clamshell type machines, for example (with the mold blocks supported on a carriage on one side), any forces incurred on the other side of the mold blocks naturally involves a larger moment arm about the carriages, with the result being that such structures can be more susceptible to "sawtooth" type arrangements through the mold tunnel, with successive mold blocks being tilted slightly rather than perfectly aligned with the axis of the mold tunnel.
Further, cost effectiveness of the machines making larger diameter tubes is a significant factor, since the size and cost of each mold block is significantly higher than the cost of mold blocks used to make small tubes. Since numerous such mold blocks are required to operate any machine to make a particular size tube, and since numerous sets of mold blocks are typically necessary to allow a machine to be used to make the desired different size tubes, the cost of each mold block and the number of mold blocks required to be on hand can significantly impact the cost of the machine and, therefore, the cost of the molded tubes. Further, since the cost of mold blocks is typically a significant cost which must be incurred up front (i.e., when the machine is first purchased), its cost must be recouped over time as the tubes are made and sold. Therefore, not only does the significant cost of mold blocks alone impact the cost of molded tubes, but the cost of carrying that expense over several years before it can be recouped can also add to the cost of the molded tubes.
The present invention is directed toward overcoming one or more of the problems set forth above.